libgimpwidgets: add "layout" property to GimpIntComboBox

The "layout" proerty controls the combo-box layout (but not its
menu layout), and can be one of:

  - ICON_ONLY:  Only show icons.

  - ABBREVIATED:  Show icons and abbreviated labels (or full
    labels, when there is no abbreviation).

  - FULL:  Show icons and full labels.  Default.

Avoid reconstructing the combo's cell-layout when the menu is
shown/hidden, by maintaining the combo's cell-layout and the menu's
cell-layout separately (probably a terrible hack, but one we already
use :P).